To my knowledge, the game's code cannot support visiting more than one player at a time. This is why you get a loading bar when you visit a city, or even when you click the aid button we already have. This feature would also clash with what's already in the game: The event popups showing what you aided, being able to see what goods you received via a Dynamic Tower bonus, as well as the way BPs are rewarded already being a bit clunky when multiple are received at once.
But hey, who cares about any of that for the sake of 'convenience'? The aid we have is good enough. Be thankful that you don't have to manually visit everybody anymore like before aid was put in!